1. Field of the Invention
The present invention relates generally to the manufacture of integrated circuits and more particularly, to a seal structure for a semiconductor die that improves substrate isolation.
2. Description of the Prior Art
Most semiconductor integrated circuit manufacturers require a seal ring at the periphery of the semiconductor die. The seal ring circumscribes the periphery of the semiconductor die and is in contact with the die. The seal ring surrounds the bonding pads, which are typically disposed along the outer edges of the die. The seal ring creates a barrier to penetration of moisture, corrosive gasses, and chemicals.
A conventional seal ring is a multi-layer structure composed of alternating conducting and insulating layers. Through each of the insulating layers run multiple vias which provide electrical paths between adjacent metal layers. The lowest layer of metal in the seal structure makes electrical contact with the substrate, which can be either p-type or n-type. This structure ensures that no oxide path is presented to the edges of the semiconductor die after the semiconductor die has been diced (i.e., cut into individual dies from a wafer).
While this seal structure provides an effective barrier to undesirable environmental effects, it presents a problem that can be severe in certain circuit applications. Specifically, the seal ring creates a substrate short-circuit path all the way around the semiconductor die because multiple sections of the semiconductor die make electrical contact with the seal ring. In the case of mixed signal integrated circuits, which integrate both analog and digital circuits on the same substrate, circuit designers take careful measures to isolate the noisy portions of the substrate that include the digital circuitry from the section of the substrate that carries the noise sensitive analog circuitry. This is usually accomplished by placing substrate barriers and substrate islands around different parts of the circuitry in order to isolate the more noisy circuit areas of the substrate from other areas that are more noise sensitive.
Isolating different circuitry areas on the substrate is effective in terms of directly isolating various portions of the circuitry from each other, but it does not address the short-circuit path between different portions of the circuitry provided by the seal ring. The seal ring provides a very low resistance metal path between different areas of the circuitry around the periphery of the substrate. Providing this path on which noise can travel from noisy areas of circuitry to noise sensitive areas defeats the attempts made to directly isolate the noisy digital areas of the substrate from the more noise sensitive analog areas.